Recent advances in bone biology have revolutionized dentistry. With better understanding of how osteoblasts respond to bone-forming signals, dentists now can use a variety of grafting materials to perform bone augmentation procedures and grow bone "de novo". In contrast, it is still largely unknown how osteoclasts regulate their activity in response to local factors. This knowledge is of great importance to the dental profession since so many dental diseases are directly or indirectly the result of unwanted osteoclast activation. One critical regulator of osteoclast activity is Microphthalmia transcription factor (Mitt). Mice carrying semidominant Mitf alleles exhibit various degrees of osteopetrosis, indicating Mitf is a promising tool for studying gene regulation in osteoclasts. The objective of the proposed study is to gain knowledge of the transcriptional regulation of osteoclasts by Mitf and three other family members, TFEB, TFEC and TFE3 (the four proteins form the "MiT" family). Since Mitf semidominant mutant mice exhibit an age-dependent phenotype, the chronological expression profiles of MiT proteins during osteoclast development will be examined since this would be consistent with functional rescue of Mitf by family members. MAPK and Rsk/PKA have previously been demonstrated to phosphorylate Mitf in melanocytes, which resulting in Mitf activation. To examine if similar processes occur in osteoclasts, recombinant MiT mutant viruses carrying disrupted MAPK and Rsk/PKA phosphorylation sites will be used, and the resulting phenotypic changes will be analyzed. Mitf has been found to regulate cytoskeletal changes in response to maturation signals during fetal osteoclast fusion. Overexpression of the individual MiT protein in fetal mi/mi osteoclasts (which do not fuse or form multinucleated giant cells) will be tested for the ability to rescue the fusion defect. Since Mitf may affect multinucleate giant cell formation by regulating the expression of molecules essential for osteoclast fusion, candidates known to be essential for this process (such as vitronectin receptor and osteopontin) will be examined for their relationship with Mitf.